It is known in the prior art to form a suspended structure in a micro-electromechanical (“MEMS”) device by forming the structure on a sacrificial material, and then removing the sacrificial material to release the suspended structure. For example, the MEMS structure may be released by etching away the sacrificial material using an etchant. The release process may require that the etchant has a high etch rate and high selectivity for the sacrificial material. Preferably, the etching process does not attack, dissolve, or destroy other exposed structure materials.
Some processes use “vapor HF” in which the etchant is hydrofluoric acid, “HF”). This is a type of “dry release” process sometimes used to avoid stiction between the suspended structure and other portions of a MEMS devices during fabrication. Thus, such structures are often coated with a passivation layer that can either remain on the structure or be removed after the release process. However, photoresist may not be a desirable material for a passivation layer in some circumstances, because it may swell after saturation by vapor HF, and may peel off of the structure being fabricated. Also, low stress nitride (“LSN”) may not be a desirable material for a passivation layer in some circumstances because it is subject to being etched by vapor HF vapor and may form residues. In addition, LSN is not easy to remove, and materials for etching LSN may attack other layers of the device being fabricated, such as oxide, silicon and metals. Among other things, LSN is not practical for use in protecting metal parts of a device being fabricated.